Current means for delivering mortar and artillery shells suffer from many sources of inaccuracy. Due to inaccuracies of measurement of the size of the propellant charge, specific firing conditions, etc., the precise trajectory of the projectile cannot always be predicted to the desired degree of accuracy. “Smart munitions” devices known in the art are generally based on GPS location systems or on pointing devices. These methods suffer from a number of obvious drawbacks. For example, a GPS signal can easily be jammed, and the use of a pointing device requires a soldier or aircraft to guide the projectile all the way to its target, exposing the soldier to enemy fire and preventing him from performing any other action.
Another approach known in the art is to use one or more sensors to measure the muzzle velocity of the projectile at or near the beginning of its trajectory. From the measurements of the initial velocity (linear and/or angular) and roll state, the exact trajectory can be calculated, and in the case of some non-ballistic projectiles (e.g. those with controllable canards (fins) or means for correcting the roll state), corrected.
Many examples of the use of sensors are known in the art. The technique of placing a plurality of magnetic sensors such as induction coils in the path of a projectile and calculating the projectile's velocity from the time delay between the signals created as the metallic projectile passes by or through the sensor has been known since the 1940's. These systems suffer from the disadvantage that the velocity measurement is performed external to the projectile, and hence any desired corrections to the projectile's trajectory must be broadcast to the projectile, leading to possible problems such as jamming of the signal, using the signal to locate the position of the control center, etc.
These problems can be obviated by, the use of an onboard sensor which measures the motion of the projectile relative to one or more markers or signal generators placed at known positions relative to the muzzle of the gun from which the projectile is fired. A few devices in which an onboard sensor is used to measure the linear muzzle velocity of a projectile at or near the commencement of its trajectory are known. U.S. Pat. No. 5,827,958 discloses a system in which a projectile contains an onboard magnetic sensor (e.g. an induction coil) and microprocessor, and two external magnets. The magnets may be attached to the gun barrel or to an external muzzle made of non-magnetic material and attached to the mouth of the gun barrel. The microprocessor calculates the muzzle velocity from the time delay between the two signals created as the projectile passes the magnets and the distance between the magnets.
German Pat. No. 3443534 discloses a projectile that contains two magnetic sensors. The time delay between the signals received by the two sensors (e.g. as the projectile exits the barrel of the gun) is measured and the muzzle velocity calculated from the time delay and the distance between the two sensors.
In addition to measurement of linear muzzle velocity, accurate knowledge of the projectile's trajectory depends on knowing its angular velocity and its roll state. A number of patents have disclosed means for measuring these quantities in flight. For example, methods are known in which the variation of the signal produced by a magnetic sensor as the projectile rotates in the earth's magnetic field is measured (e.g. Russian Patent No RU2310154). Other methods are known in which the rotations of the projectile are measured while the projectile is in flight; see, for example U.S. Pat. Nos. 5,497,704 and 7,341,221. U.S. Pat. No. 5,065,956 measures the angular velocity of a projectile using an onboard magnet sensor that compares measurements of the actual change in the magnetic field as the projectile rotates with measurements of a nominal fixed rotation.
Means for measuring the angular muzzle velocity and roll state of the projectile at the beginning of its trajectory (i.e. as it exits the muzzle) are less commonly disclosed. U.S. Pat. No. 5,233,901 discloses a method for determining the roll state of a projectile as it leaves the barrel of a gun. In this method, the projectile contains a magnet with its poles at a known angle (e.g. perpendicular) to the axis of motion of the projectile. Two windings are placed at the end of the barrel, with the windings perpendicular to one another. The roll state of the projectile as it exits the barrel is determined from the timing and phase of the signals produced as the magnet passes through the windings. The system disclosed in the patent includes the use of comparators to find the maximum signal received by each of the coils. The use of comparators can be problematic; for example, a broad but noisy peak may lead to a false calculation of the correct time at which the maximum signal was obtained. This method suffers from the additional disadvantage discussed above of depending on a signal that is produced externally to the projectile.
Thus, a system and method for determining both the linear and the angular muzzle velocities and the roll state of a projectile as it exits the muzzle of the gun from which it is fired in which the determination of these parameters is performed entirely onboard and is hence not susceptible to external interference and does not require any kind of external guidance, remains a long-felt yet unmet need.